Re: [RFC v2 13/14] virtio-net: detect pending Tx work for VQs mid-migration changes

[Eugenio Perez Martin <eperezma@redhat.com>]

On Fri, Mar 20, 2026 at 3:21 PM Jonah Palmer <jonah.palmer@oracle.com> wrote:
>
> Track per-queue pending Tx work state during early migration time.
>
> This includes a snapshot of pending Tx state for active queue pairs,
> checking for deltas at the end of migration, and freeing the snapshot
> buffer during device unrealize.
>
> With this final delta signal in place, drop the temporary always-true
> fallback in virtio_net_has_delta and return false when no deltas are
> found.
>
> Signed-off-by: Jonah Palmer <jonah.palmer@oracle.com>
> ---
>  hw/net/virtio-net.c            | 26 +++++++++++++++++++++-----
>  include/hw/virtio/virtio-net.h |  2 ++
>  2 files changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 3ee49a043a..483a43be4f 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -3943,6 +3943,14 @@ static int virtio_net_early_pre_save(void *opaque)
>      vnet_mig->mq_early = n->multiqueue;
>      vnet_mig->queue_pairs_early = n->curr_queue_pairs;
>
> +    /* Tx waiting snapshot for active queue pairs */
> +    if (!vnet_mig->tx_waiting_early) {
> +        vnet_mig->tx_waiting_early = g_new0(uint32_t, n->max_queue_pairs);
> +    }
> +    for (int i = 0; i < n->curr_queue_pairs; i++) {
> +        vnet_mig->tx_waiting_early[i] = n->vqs[i].tx_waiting;
> +    }
> +
>      /* RSS state snapshot */
>      vnet_mig->rss_enabled_early = n->rss_data.enabled;
>      vnet_mig->rss_redirect_early = n->rss_data.redirect;
> @@ -4254,6 +4262,8 @@ static void virtio_net_device_unrealize(DeviceState *dev)
>          n->migration->mtable_macs_early = NULL;
>          g_free(n->migration->vlans_early);
>          n->migration->vlans_early = NULL;
> +        g_free(n->migration->tx_waiting_early);
> +        n->migration->tx_waiting_early = NULL;
>          g_free(n->migration->rss_indirections_table_early);
>          n->migration->rss_indirections_table_early = NULL;
>          g_free(n->migration);
> @@ -4412,6 +4422,16 @@ static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
>          return true;
>      }
>
> +    /* Has any active queue's tx_waiting changed? */
> +    if (!vnet_mig->tx_waiting_early) {
> +        return true;
> +    }
> +    for (int i = 0; i < n->curr_queue_pairs; i++) {
> +        if (n->vqs[i].tx_waiting != vnet_mig->tx_waiting_early[i]) {
> +            return true;
> +        }
> +    }
> +
>      /* Has the VirtIONet's RSS state changed? */
>      if (n->rss_data.enabled != vnet_mig->rss_enabled_early ||
>          n->rss_data.redirect != vnet_mig->rss_redirect_early ||
> @@ -4439,11 +4459,7 @@ static bool virtio_net_has_delta(VirtIONet *n, VirtIODevice *vdev)
>          }
>      }
>
> -    /*
> -     * Always return true for now until we're able to detect all possible
> -     * changes to a VirtIONet device.
> -     */
> -    return true;
> +    return false;

I'm failing to see one thing: If we add a new feature that the guest
can change during migration and it's only added in the
VMStateDescription vmstate_virtio_net_device, will this "return false"
prevent the state from being resent in the stop-and-copy phase?

Mandating its addition here is ok somehow, but I'm not sure if I'm
missing something.

>  }
>
>  static bool virtio_net_needed(void *opaque)
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index 88074a0976..dbbacc83bb 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -180,6 +180,7 @@ typedef struct VirtIONetQueue {
>   * @guest_offloads_early: Guest offloads snapshot.
>   * @mq_early: Multiqueue state snapshot.
>   * @queue_pairs_early: Queue pairs snapshot.
> + * @tx_waiting_early: Per-queue pending-Tx snapshot.
>   * @rss_enabled_early: RSS enabled flag.
>   * @rss_redirect_early: RSS redirect flag.
>   * @rss_populate_hash_early: RSS populate hash flag.
> @@ -201,6 +202,7 @@ typedef struct VirtIONetMigration {
>      uint64_t guest_offloads_early;
>      int mq_early;
>      uint16_t queue_pairs_early;
> +    uint32_t *tx_waiting_early;
>      bool rss_enabled_early;
>      bool rss_redirect_early;
>      bool rss_populate_hash_early;
> --
> 2.51.0
>